Conventionally, a hydrorefining catalyst has been produced such that a carrier, which is composed of a porous inorganic oxide such as alumina, is allowed to carry an active metal which has, for example, a desulfurizing ability, a demetallation ability, and/or a hydrogenation ability. The hydrorefining is performed by contacting the hydrocarbon oil and the hydrorefining catalyst with each other in the presence of hydrogen, in which it is possible to remove the heteroelements contained in the hydrocarbon oil, i.e., sulfur, nitrogen, and metal elements (for example, vanadium, nickel, and iron). A variety of investigations have been made for the catalyst as described above in relation to the active metal, the properties of the carrier, the pore structure, and the method for carrying the active metal in order to improve the ability to remove the heteroelements.
As for the carrying solution, it has been investigated to blend, for example, metal salt, organic acid, and/or inorganic acid in order to stably dissolve the metal component. For example, Japanese Patent Application Laid-open No. 6-277520 discloses a method for producing a hydrodesulfurizing catalyst which is based on the use of a carrying solution containing a composite acid salt such as those of molybdophosphoric acid and a carbonate such as those of cobalt. Japanese Patent No. 2575168 discloses a method for producing a hydrotreating catalyst which is based on the use of a carrying solution containing molybdenum oxide, nickel carbonate, phosphoric acid, and mercaptocarboxylic acid. Japanese Patent Application Laid-open No. 4-265158 discloses a method for preparing a hydrogenation catalyst which is based on the use of a carrying solution containing cobalt carbonate, phosphoric acid, molybdenum oxide, and nitric acid. U.S. Pat. No. 3,840,472 discloses a stable carrying solution which contains a small amount of phosphorus and which is obtained by adding nickel carbonate and molybdenum oxide to a phosphoric acid solution followed by being refluxed.
The present applicant has disclosed, in Japanese Patent Application Laid-open No. 2000-42413, a method for preparing a hydrorefining catalyst which is based on the use of a carrying solution containing ammonium molybdate, cobalt carbonate, nickel carbonate, phosphoric acid, and citric acid. This carrying solution uses a relatively large amount of citric acid. For this reason, a complex, which is intended in the present invention, is not formed. As a result, the activity of the obtained catalyst is not sufficient.